Pulmonary edema is a serious medical condition caused by an excess accumulation of fluid within a patient's lungs. Pulmonary edema can be an indicator of cardiac-related diseases, such as congestive heart failure. Good management of pulmonary edema is desirable because it may allow timely therapeutic interventions, and avoid hospitalization and its costs.
It is possible to detect fluid in the lungs by making an electrical impedance measurement across the lungs. The more fluid there is in the lungs, the lower the impedance. One known way this may be done is by using an implantable medical device such as a pacemaker or defibrillator implanted in the chest area of the patient. An electrical impedance measurement is conventionally made between right ventricular chamber electrodes connected to the implanted device, and another electrode at the implanted device itself; thus, the impedance measurement samples thoracic tissues, including the lungs. This configuration may also be used to measure impedance for determining a patient's respiration rate, which may subsequently be used to aid in the regulation and issuance of pacing stimuli to the heart. For instance, a patient whose respiration rate increases due to exercise, for example, may require pacing stimuli to be delivered at a faster rate.